The upper part of a steam boiler is provided with a set of heat exchangers, in which the heat contained in flue gases is transferred to the water or water vapor flowing in a piping. A steam boiler can have e.g. eight heat exchanger banks connected in series, each of which can have e.g. 25 planar heat exchangers, between which the flue gas to be cooled flows. In the coolest heat exchanger banks, the heat transfer medium is water (feed water preheaters, i.e. economizers), in the next heat exchanger bank the water is vaporized (evaporator, i.e. steam generating bank) and in the heat exchanger banks with the highest temperature, which are closest to the furnace, the heat transfer medium is the steam which is to be superheated (superheaters).
For allowing an obstacle-free thermal expansion, the heat exchangers are usually suspended by means of hanger rods from support beams, which in turn are supported by the frame beams of the steam boiler. Fin plates may be arranged between the pipes of a heat exchanger piping bent in the form of a plane, whereby heat transfer plates are formed. Each heat exchanger bank is usually supported by two support beams provided with a number of vertical hanger rods, each of which is connected to either end of an individual heat transfer plate. The hanger rods can also support a midbeam in the direction of the support beams, or feed and discharge pipes, which support the heat transfer plates.
Fouling of the heat transfer surfaces of a steam boiler hampers the heat transfer from flue gas into the medium circulating in a heat exchanger. Recovery boilers used for the combustion of black liquor are especially prone to the fouling of heat transfer surfaces, since remarkable amounts of flue ash exit the furnace of the recovery boiler entrained in the flue gas flow, which flue ash forms onto the heat transfer surfaces deposits that are hard to remove.
Typically, impurities are removed from the heat transfer surfaces by means of a soot-blowing device, the operative steam for which is taken from the steam production of the boiler itself. Water washing can be used during shutdowns only, since, during operation, the water washing would cause thermal shocks which would stress the metal of the heat exchangers. The energy-content of the steam used for soot-blowing decreases the energy-content of the steam being transferred from the steam boiler into the turbine and other processes. Determining the soot-blowing intervals is essential for optimizing the overall efficiency of the steam boiler. The soot-blowing devices are located under various conditions and they are used based on an operation model drafted based on operational situations and experience. In the most important and most easily fouled areas, the soot-blowing devices are operated more frequently than elsewhere.
Soot-blowing maintains the efficiency and steam production capacity of the boiler by regularly removing deposited ash and slag from the heat surfaces of the boiler. Therefore, it is important to form by means of process measurements a view of the soot-blowing requirement of the boiler, so that soot-blowing can be effected in a proper part of the boiler at a proper time. By effecting the soot-blowing according to need, considerable amounts of steam can be saved and the efficiency obtained from the boiler is increased.
Monitoring the mass changes of heat exchangers for determining the optimal soot-blowing interval is known per se from publication U.S. Pat. No. 6,323,442 A, in which the mass of a heat exchanger bank suspended from hanger rods is measured by means of strain gages coupled to the hanger rods. The mass changes indicated by individual strain gages are added up, whereby the mass change of the whole heat exchanger bank is obtained. The hanger rods support the collector pipes of the inlet and outlet flows of the heat exchanger bank, which pipes in turn support the individual heat exchangers. The structure based on hanger rods is used e.g. because it is difficult to arrange an adequately firm support closer to the heat exchangers. In the example of the publication, the number of hanger rods and their strain gages is 20 for each heat exchanger bank. Additionally, the temperature of the hanger rods has to be measured for providing temperature compensation. Thus, a total of 40 measurement channels are required for each heat exchanger bank. In a subsequent publication of the same applicant, WO 2004102104 A, the corresponding measurements are made by means of load sensors from the hanger rods. Locating the load sensors elsewhere in an existing steam boiler is in practice difficult.
The large number of measurement channels is a remarkable problem for the installation of the measurement system and the equipment itself when the mass changes of several heat exchanger banks are to be measured. Thus, the large number of measurement points has a significant effect on the equipment and installation costs of the measurement system. The conditions under which strain gages are installed in hanger rods are highly disadvantageous, which increases the cost especially when the measurement arrangement is installed in an existing steam boiler as a retrofit. In order to obtain reliable results, the loads of the hanger rods have to be equalized and the measurement system has to be calibrated regularly, which is difficult in this embodiment.
A solution is known from document FR 2555740 A1 where the deformation of a frame beam bearing the load of the whole coal-fired boiler is measured in order to monitor the amount of slag in the boiler. The sensors used are vibrating wires fixed to the upper and lower flange of the frame beam and changing the vibrational frequency according to changes of the bending of the frame beam. The sensors are fixed centrally along the unsupported length of the frame beam. The sensors fixed to the upper flange of the frame beam measure the compressive strain and the sensors fixed to the lower flange of the frame beam measure the tensile strain. The sensors allow one to obtain an idea of the weight of the whole boiler, but they do not give any understanding of the weight of the different parts of the boiler.
The object of the invention is to eliminate the problems relating to the prior art solutions.